<!doctype html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <meta name="viewport"
          content="width=device-width, user-scalable=no, initial-scale=1.0, maximum-scale=1.0, minimum-scale=1.0">
    <meta http-equiv="X-UA-Compatible" content="ie=edge">
    <title>Document</title>
</head>
<body>
<div id="app">
    <input type="text" v-model="num1">
    <select name="" id="">
        <option value="0">+</option>
        <option value="1">-</option>
        <option value="2">*</option>
        <option value="3">/</option>
    </select>
    <input type="text" v-model="num2">
    <input type="button" value="=">
    <input type="text" v-model="result">
</div>
<script src="vue.js"></script>
<script>
    var vm = new Vue({
        el: '#app',
        data: {
            num1: '',
            num2: ''
        },
        computed: {
            result: function () {
                return this.num1-0+(this.num2-0)
            }
        }
    })
</script>
</body>
</html>